Method for plugging and abandoning oil and gas wells

ABSTRACT

A method and agent to induce accelerated creep deformation of shale rock formations in the annular gap between a shale formation and non-cemented sections of a casing string have been developed. A fluid containing alkali silicate or a modified alkali silicate is added to the annular space between the shale rock formation and the casing string. The alkali silicate promotes creep deformation of the shale rock, effectively closing the annulus surrounding the casing. It has been found lithium silicate provides the strongest shale-casing bond and is the presently preferred material for closing abandoned wells.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to and claims the benefit of U.S.provisional application 62/925,287 titled “Method for Plugging andAbandoning Oil and Gas Wells” filed on Oct. 24, 2019, the entirecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to fluids and methods for inducing creepdeformation in clay-rich shale rock formations. The invention results inthe deformed shale permanently closing the annular gap between thecasing and the wellbore.

Description of Related Art

Well abandonment is one of the biggest challenges in the oil and gasindustry, both in terms of cost and effort as well as the technicalchallenges associated with creating a permanent wellbore isolation. Asshown in FIG. 1, casing 10 is provided through shale layer 12. Anannulus 14 surrounds casing 10 and provides an opening for the flow ofliquids and gas from lower regions of shale layer 12 to the surface.When a well is abandoned, permanent barriers are required inside andoutside of the casing string to create zonal isolation and prevent theflow of liquids and gas through annulus 14. Current well abandonmenttechnology requires the milling and/or cutting and pulling of casingstrings to expose open hole formations, such that abandonment plugs(usually cement) can be set across them. This is a time-consuming andcostly exercise, particularly when performed offshore, because itrequires a drilling rig onsite to perform the operation.

Numerous techniques and materials have been used to form the permanentbarriers. Ordinary Portland Cement (OPC) has long been used as aplugging material. OPC as a barrier material has several shortcomings,however. OPC is brittle and does not re-heal when cracked. It is easilycontaminated by oil and oil-based drilling fluids. Furthermore, it hasrelatively low tensile strength and low strength when bonding to rockformations and casing. Moreover, the production of OPC is the secondlargest source of CO₂ emissions in the world.

The placement of OPC or cement replacement outside of the uncementedsections of casing string that have been left uncemented is an expensiveand time- and labor-intensive process, particularly offshore. Placementoften requires casing milling and pulling before abandonment plugs canbe set and often requires the use of a drilling rig, at significantadditional cost to the oil and gas well operator.

As shown in FIGS. 1-3, it is possible to use natural shale formations asbarriers in oil and gas wells for zones left uncemented behind casingstrings 10. Certain shales 12 have the ability to plastically deform and“creep” into wellbore and annular spaces 14 under certain pressure,temperature and fluid conditions. If this happens in an annular space14, this annular space 14 can be blocked off completely and a barriermay be formed that prevents migration of any gases or fluids from deepformations.

Shale represents a more effective barrier material than cement asdemonstrated by eons of isolation of oil and gas producing zones priorto drilling and production. Shales form cap rocks on top ofhydrocarbon-bearing zones, sealing them in and preventing migration ofhydrocarbons and other fluids and gases to the surface. The disadvantageto using shale as a barrier material is that it most often requires anunacceptable long time to naturally deform under in-situ rock stress andpressure. However, lowering the near-wellbore stiffness of the rockaccelerates the deformation and creep of the shale into an uncementedannular space and forms an effective barrier in that space, therebyreducing the time required for deformation.

To create an effective barrier, the shale must not only deform, but thedesirable sealing properties must not be lost in the deformationprocess. The final barrier must prevent the flow of formation gases andfluids to the surface.

It is known that rock stiffness can be manipulated and effectivelylowered by temperature increase, annular pressure reduction, and byexposure to certain annular fluids.

It would be operationally challenging and costly to apply and maintainsufficient elevated temperature across an annular space or reduce thepressure in it to induce shale deformation. The exposure of the shale toan annular fluid represents an easier and more cost effective optionthat could be done without using a drill rig.

Shales can swell, deform, and disintegrate upon exposure to differentfluids. When drilling a well, great care is taken to formulate and runappropriate drilling fluids that will stabilize shale and prevent holecollapse while drilling. Water-based drilling fluids such as alkalisilicates have been found to be effective for providing shalestabilization is alkali silicates. Alkali silicates are one of the fewwater-based fluids that can match the shale inhibition properties ofoil-based drilling fluids. Alkali silicates have been used in drillingfluids for shale stabilization, encapsulation, and prevention of lostcirculation, as well as additives in non-aqueous drilling fluids such asoil-based muds and synthetic-based muds.

The use of alkali silicates for creating annular shale barriers is notobvious. In the literature, it is known that alkali silicates canprotect shale formations and prevent them from destabilizing duringdrilling operations when such alkali silicates are used in the drillingfluid. The fact that alkali silicates can promote shale creep andaccelerate the creep deformation of shale material into an annular spaceis therefore counterintuitive

SUMMARY OF THE INVENTION

It has been discovered that alkali silicates and modified alkalisilicates offer a novel and unexpected method for inducing shale creepdeformation in shale. These alkali silicates can create a reliable, highstrength hydraulic seal. Further, these alkali silicates can inducecreep deformation and creation of a permanent seal in a wide range ofshale types.

It was unexpected that alkali silicate and modified alkali silicateswould be highly effective at inducing shale creep deformation that couldbe harnessed to expedite the formation of shale as a barrier. Theseparticular forms of alkali silicate yield a sealing material better ableto isolate pressure, gas and fluids.

It has been found that different alkali silicates perform better withvarious shale types. For instance, lithium silicate has unexpectedlybeen found to promote creep in shales and accelerated barrier formationfrom North Sea Miocene shales, and induce accelerated dispersion ofvarious Canadian shales. Lithium silicate outperformed other alkalisilicate materials for the purpose of shale annular barrier formation

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

FIG. 1 is a schematic representation showing the annular closure formedby the deformation of the shale layer surrounding a well casing.

FIG. 2 is a horizontal cross-sectional schematic representation showingthe annual closure formed by the deformation of the shale layersurrounding a well casing.

FIG. 3 is a vertical cross-sectional schematic representation showingthe annual closure formed by the deformation of the shale layersurrounding a well casing.

FIG. 4 is a schematic representation of the test set-up used inExperiment 1 to demonstrate the present invention.

FIG. 5 is a vertical cross section of the schematic representation ofthe test set-up of FIG. 4.

FIG. 6 is a schematic representation depicting the operation of the testset-up of FIG. 4.

FIG. 7 is a collection of photographs depicting the annular gap betweenthe casing string and shale layer before and after the testing describedin Experiment 1.

FIG. 8 shows the progress of the closure of the annular gap resultingfrom the shale deformed by various alkali silicates used in the testingdescribed in Experiment 1.

FIG. 9 shows the results of pressure tests performed on shale deformedby various alkali silicates used in the testing described in Experiment1.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method for placing a fluid that willinduce the deformation and creep of shale opposite subterranean rockformation. The deformed shale will have improved sealing properties dueto the presence of alkali silicate such as lithium silicate.

The annular deformation fluid comprises alkali silicates. Thesesilicates can be in the form of lithium silicate, sodium silicate,potassium silicate, or a combination. The alkali silicate is usually inliquid form but can be a dissolvable solid. An example of a suitabletype of lithium silicate is Lithisil®25.

Suitable lithium-based products such as lithium hydroxide, lithiumchloride, and lithium carbonate, may also be added to alkali silicatessuch as sodium silicate or potassium silicate. This fluid can impartaccelerated shale creep and create an effective seal.

One of the most desirable properties in an abandoned well seal is thequality of the shale-casing bond after shale creep has occurred and howmuch differential pressure it can hold. It has been found that holesclosed with lithium silicate can withstand significantly higher pressurebefore re-opening than holes closed with sodium silicate. For example,in SAAB tests, it was found that although both lithium silicate andsodium silicate had similar closure times, the re-opening pressure forsodium silicate was 450 psi and the re-opening pressure for lithiumsilicate was 943 psi. Because it produces a stronger shale-casing bond,lithium silicate is the presently preferred material for SAAB barriercreation.

The present invention will be described in connection with the followingexamples where lithium silicate has been used as the creep deformationagent. It is to be understood that sodium silicate and potassiumsilicate can also be used for other types of shale.

Experiment 1

The effectiveness of lithium silicate to expedite shale creepdeformation was demonstrated in dedicated, large-scale rock mechanicaltests. In these tests, cylindrical shale sample (1.5 inch in diameter, 3inch in length) from a North Sea shale formation were confined atdownhole stress, pressure and temperature conditions. FIGS. 4-6 depictthe equipment that was used in these tests. As shown in FIGS. 4 and 5,an artificial casing string was placed on the inside of the shalecylinder, leaving an annular gap between the rock and the casing. Thisset-up represents a miniaturized version of the actual situation in thefield, with a casing string being left uncemented opposite a shaleformation with an annular clearance that is filled in by creepdeformation of the shale (FIG. 7). Using radial strain measurements andpressure pulse decay cycles, the closure of the annular gap wascharacterized throughout the experiments (FIG. 8). At the end of thetest, the barrier quality of the shale material was probed by raisingthe downstream pressure and observing breakthrough on the upstream side(FIG. 9). The results that were obtained when varying the annular fluidchemistry and experimental conditions are set forth in Table 1 below:

TABLE 1 Breakthrough Experimental Conditions Annular Closure TimePressure Annular gap filled with artificial pore fluid, 18.4 days  Nottested temperature 55° C. Annular gap filled with 2M NaOH fluid (pH =12), No annular closure Not observed (open temperature 55° C. observedannular space) Annular gap filled with artificial pore fluid, 11.5 days 724 psi temperature 85° C. (elevated temperature) Annular gap filledwith 10% v/v Lithium Silicate 3.8 days 943 psi solution, temperature 55°C. Annular gap filled with 10% v/v Sodium Silicate 5.1 days 1054 psi solution, temperature 55° C.

The results of this experiment show that the use of alkali silicatessuch as lithium silicate and sodium silicate substantially reduces thetime required for the shale to deform and fill the annular gap aroundthe well casing as compared with other fluids at the same or highertemperatures. The seal that was formed with the lithium silicate andsodium silicate were stronger (with a breakthrough pressure of 943 psiand 1054 psi respectively, approximately equal within the ±100 psiaccuracy of the test) than that achieved through the use of elevatedtemperature (with a breakthrough pressure of only 724 psi). The sealwith the lithium silicate formed more quickly (3.8 days) than that withthe sodium silicate (5.1 days), with the elevated temperature (11.5days) and in the base test with artificial pore fluid (18.4 days).

Experiment 2

The impact of lithium on shale was compared against conventional sodiumsilicate and potassium silicate using commonly drilled dispersive typeshale found in Western Canada. Shale dispersion was measured by placing30 g of the indicated shale type in a low concentration of the indicatedalkali silicate that was diluted volume-to-volume with tap water. Thesamples were hot rolled for 16 hours at 49° C. (120° F.). After hotrolling, the samples were screened through a 16 mesh filter and rinsedwith tap water. Samples were dried and weighed.

Table 2 below shows the shale recovery after hot rolling.

TABLE 2 Potassium Potassium Silicate % vol/vol Silicate (Kasil ® 1 +LiOH) Sodium Silicate Lithium Silicate Shale Name alkali silicate/waterKasil ® 1 (95:5 wt/wt) N ® grade Lithisil ®25 Shaftesbury 2.5% 47.3 62.839.7 35 Joli Fou 2.5% 51.7 46.0 21. 3.3 Lea Park 5.0% 48.7 10.1 6.2 0White Spec 5.0% 11.3 49.3 6.3 0

For drilling purposes, it is imperative that shales are stabilized, andhence obtaining high values for the shale recovery percentage duringhot-rolling is considered advantageous. However, for theshale-as-a-barrier application, obtaining lower numbers is morebeneficial, as it will lead to a larger strength reduction, andconcomitantly a higher creep rate, of the shale material. It is clearfrom Table 2 above that lithium silicate has the lowest shale recoverypercentages of the various materials tested, and is the most suitablematerial to induce enhanced shale creep.

Various modifications and alterations to this invention will becomeapparent to those skilled in the art. The amount of bentonite in thethermoplastic composition can be adjusted to modify the “clump” force,suitable to avoid clumping of the pellets in any adverse storageconditions.

While the above description contains certain specifics, these should notbe construed as limitations on the scope of the invention, but rather asan exemplification of one preferred embodiment thereof. Accordingly, thescope of the present invention should be determined not by theembodiment(s) illustrated, but by the appended claims and their legalequivalents.

We claim:
 1. A method for promoting creep deformation of shalesurrounding a well casing comprising the step of introducing alithium-based product into an annulus between said shale and said wellcasing, whereby said lithium-based product promotes the creepdeformation of the shale.
 2. The method of claim 1 wherein saidlithium-based product is introduced in aqueous form.
 3. The method ofclaim 2 wherein said lithium-based product is lithium silicate.
 4. Themethod of claim 2 wherein said creep deformation of the shale fullycloses said annulus.
 5. The method of claim 2 wherein said lithium-basedproduct is added to sodium silicate.
 6. The method of claim 5 whereinsaid lithium-based product is a modified lithium silicate.
 7. The methodof claim 5 wherein said lithium-based product is a lithium hydroxide. 8.The method of claim 5 wherein said lithium-based product is a lithiumcarbonate.
 9. The method of claim 5 wherein said lithium-based productis a lithium chloride.